Two areas in mammary gland biology are under investigation 1) transcription elements and signalling pathways that regulate the activation of milk proteins genes; 2) genetic switches that facilitate mammary gland development and differentiation. The whey acidic protein (WAP) gene serves as a paradigm to understand gene regulation in the mammary gland. Transcription elements and signalling pathways have been identified that control activation of this gene during pregnancy. We have cloned and analyzed a new member of the STAT5 (signal transducers and activators of transcription) family of proteins, that mediate prolactin induced transcription. Current efforts focus on the roles of Stat5 members in mammary development through the deletion of their genes by homologous recombination. Genetic switches and pathways are being investigated that mediate normal and oncogenic development of mammary tissue during puberty, pregnancy, lactation and involution. Genes have been identified which are critical for normal mammary gland development, and their role is being studied through over-expression in mammary tissue of transgenic animals and through their deletion from the mouse genome by homologous recombination. The effects of tumor growth factor (TGF) beta 1, TGF- alpha and the Notch related cell fate protein Int3, insulin like growth factor (IGF)I, and p53 and pRb on mammary development have been analyzed. The imbalanced expression of these molecules specifically alters development, differentiation and involution of the gland. We are currently focusing on the mechanisms by which these molecules, as well as inhibins and oxytocin control mammary development. Within a few days after weaning the entire alveolar compartment of the mammary tissue is remodelled by means of extensive apoptosis. Although p53 has been shown to be critical for apoptosis in many cell systems, apoptosis in mammary tissue occurs and proceeds in the absence of p53, and is probably mediated by members of the bcl-2 family. This was shown in with transgenic mice in which p53 was inactivated by the SV40T antigen, and in p53 -/- mice. A thorough understanding of development and disease depends on a temporally controlled activation of genes. This system is currently being applied to inactivate cell cycle proteins for defined time periods. This should allow us to answer old questions in developmental and tumor biology. In particular, it will allow us to identify time frames in tumor progression during which oncogenic hits accumulate. Additional information conducted in this laboratory can be found in the Biology of the Mammary Gland Web site (http://scrc.dcrt.nih.gov./~mammary/Groups/Hennighausen/Hennighausen- _group.html).